The present invention relates to a method and an arrangement for detecting the presence of an ionizing current in an ignition circuit incorporated in the ignition system of an internal combustion engine, in which a measuring voltage is applied to at least a secondary winding in the ignition circuit, and in which a measuring device is used to detect the possible presence of ionizing current in the ignition circuit.
Such a method and arrangement are described in Swedish Patent Specification SE No. 442 345. This specification describes the use of an essentially constant measuring voltage for detecting or establishing the presence of ionizing current, for the purpose of detecting abnormal combustion and/or of establishing in which cylinder combustion has actually taken place. In order to establish positively that combustion is abnormal, e.g. to establish knocking, the measuring voltage applied to the electrodes of the spark plugs is preferably relatively high, e.g. in the order of 400 volts. When using capacitive ignition systems, the ignition voltage is normally stepped up in two stages, a voltage of the foregoing magnitude being obtained in an intermediate stage.
Thus, in known ignition systems of this kind it is very easy to obtain the measuring voltage required to detect the ionizing current.
The use of a relatively high measuring voltage, however, has disadvantages. Tests have shown that the spark plugs quickly become coated with soot particles which are liable to prevent the engine from starting, particularly in the course of starting an engine, and especially when the engine is cold.
When the sole purpose of measuring ionizing current is to establish whether or not combustion has actually taken place, it is possible to use a lower measuring voltage, in the order of 60-100 volts. However, although this lower voltage will reduce the extent to which carbon deposits are formed on the spark plugs and therewith alleviate the problems of ignition, the application of a lower voltage is itself accompanied by certain drawbacks. When detection of the possible presence of an ionizing current is effected in order to establish whether combustion is abnormal or not, preferably to detect the occurrence of knocking and premature ignition, it is safest to utilize a high measuring voltage, with which the foregoing drawbacks cannot be avoided.
The reason why spark plugs become sooted up in this fashion is because the soot, or carbon, particles are charged electrically and consequently are attracted to the electric poles constituted by the spark plug electrodes in an ionizing current measuring process. The same physical properties are utilized purposely in so-called electrostatic filters, in which a voltage field generated between two poles is utilized to filter out solids present in such field.
Soot particles present in the combustion chamber consist essentially of non-combusted fuel. Normally, when starting an engine, an excess of fuel is supplied to the engine in order to facilitate the start. This means that the number of soot particles produced will also increase, thereby aggravating the problem of measuring ionizing current.
The object of the present invention is to avoid this drawback while, nevertheless, ensuring that the ionizing current is measured reliably.
This object is achieved with the inventive method in which a parameter representing an engine start sequence is detected. When an engine start sequence is detected, one of a first substantially constant measuring voltage or no measuring voltage is applied to an ignition circuit in at least one secondary winding. Responsive to such measuring voltage, a measuring device is used to detect the possible presence of ionizing current in the ignition circuit. When detection of the engine start sequence is terminated, a second, substantially constant measuring voltage is applied. The first measuring voltage is lower than the second measuring voltage.
The invention also relates to an arrangement for carrying out the inventive method. In the arrangement there is connected to a connection of an ignition circuit and an external voltage source a connection to earth which includes a semiconductor switch. Such switch receives control signals from a control unit connected to transducers for sensing at least one engine parameter used for detecting an engine start. The control unit is configured to send a signal to turn on the semiconductor switch for closing the earth connection during an engine start; accordingly, the measuring device will apply a lower voltage than that applied in the absence of an engine start.
Thus, the use of a very low measuring voltage, or the total omission of a measuring voltage, during an engine start will avoid or at least greatly alleviate the above-mentioned problem. Although it is not possible to detect reliably the possible presence or occurrence of an ionizing current with the aid of a low measuring voltage, which in itself create drawbacks, this problem can be readily overcome, as will become apparent from the following description.
In a computer-controlled ignition system which lacks a mechanical high voltage distributor, it is possible to utilize an established normal combustion process as a starting point for triggering the supply of ignition voltage to respective cylinders in a given sequence for continued operation or running of the engine. This obviates the need to identify respective cylinders with the aid of cam shaft sensors, as in the case of conventional solutions. In order to remove the drawback created by the invention, it is necessary in the case of a computer controlled ignition system to initiate ignition each time a cylinder is located in its top dead-centre-position during an engine start. In the case of a four-stroke engine this means that, when starting, the engine ignition is initiated twice during a combustion cycle instead of once. A computer controlled ignition system solely requires a modified program having no need for additional components. The resulting increase in wear on the spark plugs can also be overlooked in the present context, since an engine starting sequence is normally of very short duration.
It is not possible when practising the invention to detect that combustion is not normal, e.g. that knocking has developed. However, abnormal combustion only occurs when the engine is hot and/or is heavily loaded. Since such operating conditions do not ococur during a normal engine starting process, the fact that abnormal combustion cannot be detected by means of the inventive method or arrangement constitutes no disadvantage.
When an engine has been started and has run for some time so as to become hot, the sparks which occur normally across the spark plug electrodes will continuously burn off any soot or carbon deposits that may form. It is therefore possible to increase the measuring voltage to a higher level of which the presence of ionizing currents can be detected in a known manner.
The invention thus enables the advantages afforded by an ionizing current measuring process while eliminating the drawbacks which such measuring processes create during an engine start.